1. Field of the Invention
The present invention relates to a radio transmitting method and an apparatus, and a radio receiving method and an apparatus in a radio communication system.
2. Description of the Related Art
A scheme, which improves receiving performance by multiplying a signal by a weight for each antenna in transmitting signals using a plurality of antennas, has been well known.
For instance, a scheme, which obtains desired gains by multiplying each signal to be transmitted from a transmission side by a weight so as to make a phase and an amplitude of the reception signal from each antenna equal to one another at a receiving end in transmitting the identical signals from a plurality of antennas, has been known. A scheme, which easily separates a plurality of signals by multiplying weights for adjusting phases and amplitudes on a transmission side so that signals overlapped with a plurality of channels at a receiving end easily become orthogonal to one another in transmitting a plurality of signals from a plurality of antennas, has been known. Generally, in these schemes, the finer the adjustment that can be applied to the weights, the higher the performance obtained.
In a system such as a time division duplex (TDD) system using the same frequency on an uplink and a downlink, the channels of the uplink and the downlink are almost same as each other. Therefore, for example, the TDD system may predict a channel from the signal to be received on the uplink and compute a weight sequence to be used on the downlink on the basis of the channel estimation result.
In contrast, in a system such as a frequency division duplex (FDD) using different frequencies on the uplink and the downlink, generally, the channels of the uplink and the downlink are different from each other. Therefore, for example, the FDD system may not compute a weight sequence to be used on the downlink from the signal to be received on the uplink on the basis of the channel estimation result. Accordingly, it is needed for the FDD system to compute a desired weight sequence by predicting the channel of the downlink on a reception side of the downlink and send weight sequence indicator information indicating the computed weight sequence to a transmission side through the uplink.
3GPP TS 25.214 v3.12.0 (2003-03), Mar. 26, 2003 (Document 1) prepares a plurality of kinds of weight sequences indicating the weight corresponding to each of two antennas. The plurality of kinds of weight sequences are called a weight sequence set. This weight sequence set is shared on both the transmission and reception sides. According to Document 1, on the reception side, one weight sequence is selected from the weight sequence set, and an index for the selected weight sequence is sent to the transmission side as weight sequence indicator information [called a feedback signaling message (FSM) in Document 1].
To finely adjust the weight and improve the receiving performance, it is needed to make the size of the weight sequence set larger. In sharing the weight sequence set by both the transmission and reception sides as disclosed in Document 1, when the size of the sequence set becomes large, such a problem of an increase in overhead of the weight sequence indicator information is produced.